The present invention relates to the field of optical filters.
One of the main applications of optical filters is their use as gain flattening filters in optical amplifiers. These amplifiers do not amplify all wavelength equally because they have a wavelength-dependent gain. As a result, some wavelengths are more amplified than others which means that the weaker signals may get lost in the noise. A gain equalizing filter can compensate for this difference by restoring all wavelengths to approximately the same intensity.
An erbium doped fiber amplifier (EDFA), for example, is one of the key devices for supporting WDM systems. The gain spectrum of an EDFA has asymmetrical twin peaks and a non-flat output response resulting in a power deviation between amplified signals. Furthermore, in long haul optical transmission systems, optical signals are transmitted by multi-amplifier systems, so distortions of the signal after multiple passes through optical amplifiers do occur. As a result, optical signals with a low signal to noise ratio are increasingly deteriorated in a WDM system. Thus, the transmission distance becomes shorter and the wavelength range of transmittance and the number of signal channels decrease. In order to obtain an adequate signal to noise ratio at each wavelength, it is necessary to flatten the gain of the amplifier in the range of the signals. One of the methods to flatten the gain of an amplifier is using an optical gain flattening filter. This method flattens the gain by using a filter with a reverse loss spectrum against the gain spectrum of the amplifier. It is normally desirable to maintain a flat spectral response within each channel in the system so that different wavelengths undergo similar gain or attenuation when passing through various stages of a communication system.
Conventional gain flattening filter technologies include thin film filters, short period fiber Bragg gratings, and fused fiber couplers. However, more recent trends in the design of gain flattening filters move towards customer tunable and variable gain equalizers. One example of a tunable gain flattening filter is an etalon-type gain flattening filter consisting of several etalon filters with different amplitudes and phases to compensate asymmetric amplifier gain-wavelength characteristics for flattening gain shapes. A fine control of thickness and reflectance of each etalon filter is needed to obtain each amplitude and phase of etalon filters. However, etalon-type gain flattening filters are rather expensive and not compact in size.
A variety of spectrum equalizing (or flattening) techniques have been developed to address such problems. For example, in U.S. Pat. Nos. 5,532,870 and 5,640,269, Shigematsu et al. disclose an optical fiber amplifier which reduces the wavelength dependency of gain in various wavelength ranges in wavelength division multiplexing transmission by using at least two kinds of optical fibers serially coupled, each having a glass composition selected from at least two kinds of rare-earth-doped glass compositions. Another example is the optical amplifier disclosed by Minelly and Laming in U.S. Pat. No. 5,526,175, which amplifies signals of different wavelengths throughout a spectral window while equalizing the output levels of the signals, by using a dichroic reflector at one end of an amplifying fiber to set up standing wave patterns therein by interference of the forward and reflected signal lights, at the different wavelengths. Furthermore, daSilva et al. disclose in U.S. Pat. No. 5,345,332 a technique for channel-by-channel power regulation in a multiwavelength lightwave communications system by using a cascade of inhomogeneously broadened saturated fiber amplifiers spaced along the optical fiber transmission path.
The above disclosed techniques, however, are either too complex or more applicable to band-limited optical communication systems and may, therefore, present expensive solutions for systems transmitting a relatively large number of multiplexed channels. Such a problem is of a particular concern in optical communication systems where the selection of equalizing filters is more limited than in traditional radio communication systems. There is, therefore, clearly an important need for more economical solutions for equalizing wavelength-division multiplexed channels, especially in the case of lightwave communications, as well as a need for tunable gain flattening filters.
Furthermore, the above described applications are limited to gain equalization. There is a need for optical filters having an output response with a complex shape to yield a predetermined output response with asymmetric characteristics, i.e. other than square or sinusoidal output responses.
Carlsen discloses a birefringent optical multiplexer having a flattened bandpass in U.S. Pat. No. 4,685,773, incorporated herein by reference. Carlsen""s multiplexer system utilizes birefringent optical filtering in combination with a polarization insensitivity feature and the ability to tune the passband. However, the ability to tune this multiplexer system is restricted to integer multiples of the output responses of the individual birefringent optical filtering elements.
It is an object of this invention to provide an improved tunable optical filter.
Another object of this invention is to provide an tunable optical filter that is compact in size and cost effective.
It is a further object of the invention to provide an optical filter which is tunable to fit a plurality of gain spectra.
A further object of the invention is to provide an optical filter having an output response with a complex shape.
In accordance with the invention there is provided an optical filter comprising a first wave retarding means for receiving an optical signal including a plurality of wavelengths, said first wave retarding means having a first periodic output response of a first phase versus wavelength for the optical signal, said optical signal being polarized when being launched into the first wave retarding means; and a second wave retarding means for receiving the optical signal from the first wave retarding means and having a second periodic output response of a second phase versus wavelength for the optical signal, and wherein the first periodic output response and the second periodic output response are different from each other and non-integer multiples of each other.
In accordance with another embodiment of the invention, a period of the first output response and a period of the second output response differ by at least 0.5 nm.
In accordance with a further embodiment of the invention, the optical filter further comprises a first polarizing beam splitter for providing the polarized optical signal launched into the first wave retarding means by splitting an input optical signal into two orthogonal sub-signals, and a second polarizing beam splitter for receiving the two sub-signals from the second wave retarding means and for combining the two sub-signals.
The present invention further provides an optical filter wherein the first and the second wave retarding means are independently rotatable about an optical axis of the optical filter for tuning the optical filter.
Thus, in accordance with some embodiments of the present invention a high order waveplate based optical filter is capable of producing any desired spectral curve without polarization dependent losses by simply adjusting the angle of each high order waveplate.
Polarization mode dispersion and broadening is eliminated by an orthogonal two-stage configuration of a first and a second filter assembly.
In accordance with the invention, there is further provided, an optical spectral equalization system for equalizing amplitudes within each channel of a group of multiplexed channels having different predetermined central wavelengths, said system comprising: a plurality of high order waveplates each having a periodic output response of a phase versus wavelength for a beam of light corresponding to the group of multiplexed channels, wherein the periodic output response of each of the plurality of high order waveplates is different and a non-integer multiple of the respective other output responses.
The invention further provides an optical spectral equalization wherein a sum of the periodic output responses of the plurality of high order waveplates is a non-periodic response of a phase versus wavelength having a complex shape for serving as an equalizer to the beam of light, the beam of light having a complementary complex shape such that an output of the optical spectral equalization system is substantially flat.
In accordance with another aspect of the invention, there is provided, a method of determining a number, length, material, and orientation of a number of wave retarding means in a gain equalization system for yielding a predetermined non-periodic output response for a beam of light having multiple wavelengths passing through the plurality of wave retarding means comprising the steps of: determining the predetermined non-periodic output response of a phase versus wavelength; performing a Fourier series analysis for determining a number of periodic output responses of a phase versus wavelength whose sum corresponds to the predetermined non-periodic output response; determining the number of wave retarding means from the number of periodic output responses; and determining the length, material, and orientation of each of the number of wave retarding means from each periodic output response of the number of periodic output responses.
In accordance with some embodiments of the invention, the period of the wave retarding means is varied by varying the length of the wave retarding means, and the amplitude of the wave retarding means is varied by rotating the wave retarding means about an optical axis of the gain equalization system.
In comparison to thin film coating or fiber Bragg grating methods, for example, a gain flattening filter in accordance with the present invention advantageously provides tunability by adjusting the angles of the high order waveplates. In comparison to acoustic and tilting Fabry-Perot cavity tuning methods, the advantages of the method and apparatus in accordance with the present invention include compact size, stability, and low cost. The combination of these advantages make the apparatus and method in accordance with the present invention not only ideal for low cost compact tunable filter applications but also for pre-adjusted fixed filter applications which fit a given spectral curve using standard components.